Smart solid state relay

ABSTRACT

An electrical system includes a solid state relay ( 1 ) and an electrical connector ( 2 ) that connects solid state relay ( 1 ) to a load ( 4 ). The solid state relay ( 1 ) includes a power MOSFET (Q 1 ) for switching power to the load ( 4 ). A PNP transistor (Q 2 ) monitors the voltage drop across the power MOSFET (Q 1 ), and shuts the power MOSFET off when the voltage drop exceeds a reference level. The solid state relay circuitry floats when the power MOSFET is commanded OFF so there is no leakage to ground. The relay ( 1 ) can be used with an electrical connector that includes a short pin ( 34 ) or shunt ( 16 ) that is disconnected before male and female terminals ( 12, 22 ) are unmated. Disconnection of the shunt ( 16 ) or the short pin ( 34 ) causes the power MOSFET to be commanded OFF so that there is no current flowing through the male and female terminals ( 12, 22 ) when they reach an arc susceptible position. The solid state relay ( 1 ) and the connector ( 2 ) are suitable for use in a 42 Volt automotive electrical system.

REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 10/068,925filed on Feb. 8, 2002 now U.S. Pat. 6,891,705 which is herebyincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is related to a solid state relay including a powerMOSFET in which the circuitry of the relay floats when the power MOSFETis commanded OFF to prevent leakage currents from draining a battery.This invention is also related to the prevention of arcing whenelectrical connectors are unmated. This invention is also related to 42volt electrical systems that can be used in automobiles or motorvehicles to reduce electrical losses.

2. Description of the Prior Art

Contacts carrying significant amounts of power will arc whendisconnected. The amount of arc damage experienced by the contactsdepends on their physical structure, the load current, the supplyvoltage, the speed of separation, the characteristics of the load(resistive, capacitive, inductive) as well as other factors.

Future automotive systems are expected to utilize 42 volts in order toreduce the load currents and the associated wiring losses. Thisincreased voltage could cause significant arc damage to occur to thepresent connectors designed for 12-volt operation. To avoid the possibleliabilities associated with catastrophic connector failure, automotivemanufacturers are requesting a new connector design that can behot-swapped some significant number of times. Twenty cycles isconsidered to be a minimum requirement.

To disconnect 42-volt power without significant damage requiresinterrupting about 1500-watts for many loads and as much as 15 KW forthe main battery circuit. Present day modules used in automotiveapplications can consume more than 500 watts. Power supplies mustdeliver one or more kilowatts of energy. Conventional solutions requireeither that the current be shut off before the contacts are separated orunmated or employ a sacrificial contact portion. Cost, space,reliability, safety, performance and complexity of these conventionalsolutions make them unsuitable for applications, such as automotiveelectrical systems.

There are many things, known in the power utility profession, which willquickly extinguish an arc and many things known in the relay industrythat will minimize arc damage to connectors and contacts. These can befound in literature such as Gaseous Conductors by James D. Cobine andthe Ney Contact Manual by Kenneth E. Pitney. Most of these methods arenot practical in typical smaller and separable electrical connectorssuch as those used in automobiles, computers and appliances. None ofthem will eliminate arcing. In fact, even contacts that are rated forcurrent interruption use in such smaller power connectors will bedestroyed by interrupting rated currents often enough or slowly enough.There is a finite life for existing connectors since arcing will occurand cause damage each time the connector is disconnected.

One approach that has been suggested is to include a relay in theelectrical system that will be switched off prior to disconnection of anelectrical connector. The relay could be incorporated in a junction boxor other enclosure that must be opened before the connector can bedisconnected. When the junction box or enclosure is opened, the relaywould also be opened when this approach is employed. Such an approachwould, however, require additional components for every electricalconnector that might be unmated or mated under load, and as such wouldadd complexity and cost to an automotive electrical system.

Another alternative that has been considered is to incorporate aswitching component, such as a power MOSFET, in an electrical connector.Such a switching device would be switched off before arcing could occur.However, individual power MOSFET's do not possess the requiredcombination of size, current carrying capacity and cost to make such asolution practical at the present time. In addition conventional powerMOSFET's have not been widely accepted for use in automotiveapplications, because of leakage currents that can drain a battery whena large number of such devices are used in an automotive electricalsystem. U.S. patent application Ser. No. 5,926,354 discloses a solidstate relay and a circuit breaker that includes a power MOSFET. However,the solid state relay circuit disclosed therein includes a groundconnection through which current can leak from the battery positiveterminal to ground when the power MOSFET is commanded to the OFF state.It is believed that conventional solid state relays that employ a powerMOSFET exhibit this leakage problem if those relays are used in astandard relay package with a standard pin configuration. A solid staterelay in accordance with the instant invention eliminates the leakageproblem for solid state relays in standard configurations.

SUMMARY OF THE INVENTION

The instant invention comprises a solid state relay including a powerMOSFET in which there is no leakage to ground when the solid state relayis powered, but commanded to the off state, in which current is notcarried by the power MOSFET. Furthermore, this solid state relay can beused in conjunction with an electrical connector in which the solidstate relay will be turned off between the start of unmating of matableelectrical connector halves and complete unrating of the two connectorhalves.

A solid state relay, according to this invention includes a power MOSFETfor switching current to a load. The MOSFET includes a source, a gateand a drain. The power MOSFET is turned on by an active low input to thegate. The solid state relay also includes a circuit for applying a gateinput to shut off drain to source current when a voltage drop betweenthe MOSFET source and drain exceeds a reference voltage. The powerMOSFET is isolated from ground potential except through the-gate whenthe solid state relay is connected between a positive battery voltageand a load. The circuit and the active low gate are configured to floatin the absence of an active low input to the gate, and are not tied toground, to prevent leakage when the power MOSFET is in a nonconductingstate.

In representative embodiments, the solid state relay includes a firstrelay terminal connectable to a high battery voltage potential, a secondrelay terminal connectable to the high side of a load, and a third relayterminal comprising a signal input terminal. This solid state relay alsoincludes a power MOSFET having a source connected to the first relayterminal and a drain connected to the second relay terminal. The powerMOSFET includes a gate connected to the third relay terminal. The powerMOSFET is turned to an On state by an active low input applied to thethird relay terminal. A pull up resistor is connected between the firstand third relay terminals. A circuit, including a PNP transistor in thepreferred embodiment, senses source-drain voltage drop when the powerMOSFET is in an ON state. The voltage sensing circuit is connected tothe gate so that when the source-drain voltage drop exceeds a referencelevel, the power MOSFET is turned to an OFF state. The solid state relaycircuitry floats relative to ground potential when the active low inputis removed from the third terminal so that leakage current between ahigh battery voltage potential and a ground voltage potential iseliminated when the power MOSFET and the solid state relay are commandedto an OFF state by the absence of an active low input at the third relayterminal. This relay can be used in conjunction with an electricalconnector connecting the solid state relay to the load having a long pinto the second relay terminal and a short pin attached to an additionalrelay terminal.

According to another aspect of this invention, an electrical system,such as a 42 Volt automotive electrical system, includes an electricalconnector and a solid state relay attached to the connector to preventarcing when mating contacts in the electrical connector aredisconnected. The electrical connector includes first mating contactmeans, such as load pins or blades, and second mating contact means,such as a shunt or short pin. The first mating contact means havesufficient current carrying capacity to carry the entire current throughthe connector. The second mating contact means will be disengaged priorto disconnection of the first mating contact means, when the electricalconnector halves forming the electrical connector are unmated. The solidstate relay includes a power MOSFET. The power MOSFET is switched offwhen the second mating contact means are disconnected so that no currentis carried by the first mating contact means when the first matingcontact means are disconnected.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a circuit in which a power MOSFET is used in asolid state relay that can be switched off before arcing could occur asan electrical connector is unmated or mated.

FIG. 2 is a schematic similar to FIG. 1 in which two power MOSFET's areparalleled in the smart solid state relay.

FIG. 3 is a view showing the relative position of electrical contacts inan electrical connector in a fully mated configuration.

FIG. 4 is a view of the connector of FIG. 3 during initial unmating ofconnector halves, showing the manner in which a shunt is disengagedbefore terminals carrying the electrical load are disconnected.

FIG. 5 is a view of the connector configuration of FIGS. 3 and 4, inwhich male and female load carrying terminals are in a position wherearcing might occur if the relay had not previously opened this circuitas a result of disconnection of the shunt.

FIG. 6 is a view showing male and female terminal unmated, but with theshunt engaging the disengaged male terminal.

FIGS. 7A and 7B are views of male and female terminals in an alternateembodiment of this invention in which a short terminal is disconnectedbefore longer load terminals to allow sufficient time for the smartsolid relay to switch off. FIG. 7A shows the connector in which bothlong and short pins are mated and FIG. 7B shows that the short pin isunmated before the long or load pin.

FIG. 8 is a view showing one version of the solid state relay inaccordance with this invention. This embodiment of the relay uses quickconnect terminals of the type conforming to ISO 7588-2.

FIG. 9 is an exploded view of the solid state relay, also shown in FIG.8, showing two power MOSFET's used in this relay.

FIG. 10 is a schematic of an embodiment of a solid state relay inaccordance with this invention that can be substituted for aconventional electromechanical relay.

FIG. 11 is a schematic of another embodiment of a solid state relay inaccordance with this invention. The embodiments of both FIGS. 10 and 11are suitable for use in applications other than the prevention of arcingas an electrical connector is unmated or mated.

FIG. 12 is a view of a pin configuration for a standard mini relay thatconforms to ISO 7588-2.

FIG. 13 is a view of a pin configuration for a standard mini relay thatconforms to SAE J1744.

FIG. 14 is a schematic of a standard Form A relay.

FIG. 15 is a schematic of a standard relay having two switched contactsat the same voltage potential.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the solid state relays in accordance withthis invention are intended to substantially conform to conventionalstandards for electromechanical relays, such as ISO 7588-2 or SAE J1744.Although these specific standards are discussed with reference to therepresentative embodiments of the invention, it should be understoodthat the elements of this inventions could be adapted to other standardor custom relay configurations. Some preferred embodiments are thereforesuitable for replacement of standard electromechanical relays, withoutintroducing current leakage common to devices employing power MOSFET'sas the primary switching devices. Other preferred embodiment can be usedin other applications, such as to prevent arcing when an electricalconnector is mated or unmated, but these other embodiments still conformin many ways to these standards. For example, the pin configurations forthose relays that are used to prevent arcing still basically conform tostandard pin configurations so that minimal changes to other componentsof wiring systems need be made. For this reason the standard pindesignations, 30, 85, 86, 87 and 87 a are used for each embodiment. Thelocation of these pins is also shown in schematics of the relaycircuits. Each of the embodiments of solid state relay has beenidentified as solid state relay 1, even though various embodiments maydiffer in detail. Certain embodiments of this relay include componentsthat could be added as separate components to circuits in which thoseembodiments might be employed.

The first embodiment of solid state relay 1 shown in FIG. 1, is OFF (Q1not conducting) with its circuitry floating, with respect to ground,until it is commanded ON by an Active Low signal at the input terminal.The circuitry is allowed to float to eliminate leakage current flowingfrom B+ to ground when the relay 1 is powered but is commanded OFF.Leakage can be a significant problem in automotive applications in whichother solid state relays might used to switch conventional loads. Evensmall leakage currents can significantly drain a battery when a largenumber of solid state relays are used in an automobile or motor vehicle.

The solid state relay 1 shown in FIG. 1 employs a conventional powerMOSFET. In the preferred embodiment, an IRF 4905 MOSFET manufactured byInternational Rectifier is employed. This particular power MOSFET isrelatively simple and relatively inexpensive and does not include someof the additional features of other power MOSFET that are not requiredfor this solid state relay application. The source of this power MOSFETis at a higher potential than the drain when the power MOSFET is OFF. Asshown in FIG. 1, the source is connected to the positive battery voltagethrough an intervening diode D3. The power MOSFET drain is connectedthrough a connector 2 to a load, which is in turn connected to therelatively negative battery voltage. Details of this connector 2 will besubsequently discussed in greater detail. The load depicted in FIG. 1 isrepresentative of any one of a number of conventional loads, includinginductive loads, which can be switched with this relay.

The power MOSFET gate is connected to conventional control circuitrythat need not be shown or discussed in detail. It is only necessary thatthis control circuitry be capable of inputing an Active Low signal tothe power MOSFET, and in turn the solid state relay, is to be commandedON. When the solid state relay 1 is commanded ON, the power MOSFET Q1turns on and begins conducting, thus providing power to the load. Whenused in a motor vehicle application, the Active Low input signal couldbe generated as a result of a manual command to energize a particularload, such as lights or a heater fan, or the Active Low input signalcould result automatically because of a change in state of somemonitored variable.

A bipolar PNP transistor Q2 is also part of circuitry of this solidstate relay. The emitter of transistor Q2 is connected to the positivebattery voltage through diode D3. The collector of transistor Q2 isconnected to the gate of the power MOSFET at the junction of resistorsR3, and R4. The base of transistor Q2 is connected to the emitterthrough a resistor R8. The transistor base is also connected to therelay contact terminal 86 through resistors R9 and R7. The PNPtransistor base is also connected to at least one pin or contact in theconnector 2 through intervening resistor R9 and diode D2.

The purpose of the PNP transistor Q2 is to monitor or sense the voltagedrop across the source and drain of the power MOSFET Q1 and a shunt orshort pin in the connector. When the solid state relay 1 commanded ON,with the power MOSFET conducting, the drain/source resistance Rds (on)and the current will result in a voltage drop across source/drainjunction of power MOSFET Q1 that is proportional to the current flowingthrough the power MOSFET Q1. In the preferred embodiment, when thevoltage drop across the source/drain junction and the shunt or short pinconnection exceeds 0.7 volts DC, the PNP transistor Q2 turns ON,shutting off the MOSFET Q1. When this solid state relay 1 is used withthe connector 2, the diode D2 and resistors R8, R9 and R7 provide thecapability of biasing the Emitter/Base junction of the PNP transistor toshut off the power MOSFET Q1 at varying current ranges. In the preferredembodiment, the Emitter/Base junction can be biased between 0.1V and0.6V.

A number of components are included in the solid state relay 1 toprotect the relay or other components of the electrical system in whichthe relay is used. Zener diode Z1 is connected between the positive andnegative battery voltages to provide over voltage protection for thebattery. For a 14 volt system, a diode Z1 will typically be selected toprotect the battery from load dump transients which would exceed apredetermined value, such as 30 volts. For a 42 volt system, a diode Z1would be selected to protect the battery from 68 volts. Diode D3 isconnected at the positive battery voltage to protect circuitry connectedto the battery from reverse battery connection. Diode D1 is connectedbetween the negative battery voltage and a point between the load andthe drain of power MOSFET Q1. Diode D1 is a free wheeling diode thatprotects the relay 1 from transients generated by disconnection frominductive loads. The RC circuit of R4 and Cl provides immunity fromtransients, such as inrush current from lamp loads, prematurely shuttingoff power MOSFET Q1. Zener diode Z2 limits the gate voltage of the powerMOSFET Q1 and provides a constant voltage for operation of PNPtransistor Q2. Pull up resistor R2, between the gate drive circuit andpositive voltage potential prevents inadvertent triggering of the relay1.

As shown in FIG. 1, the solid state relay 1 is used in conjunction witha connector 2 in a configuration that will prevent arcing when theelectrical connector is disconnected from the load. Although arcing hasnot been regarded as a problem in many applications, such as 14 Voltautomotive electrical systems, a 42 volt electrical system could resultin arcing problems, especially when an electrical connector isdisconnected under load. At least one load pin in included in theconnector for conducting current through the power MOSFET Q1 and theload. The connector 2 also includes a short pin or a shunt. Either theshort pin or the shunt would be disconnected prior to disconnection ofthe load pin. As shown in FIG. 1, either the shunt or the short pin isconnected to the drain of the power MOSFET Q1. Since the short pin orshunt is connected indirectly to the base of the transistor Q2,disconnection of the short pin or shunt, or the drain of the powerMOSFET Q1, will cause the PNP transistor Q2 to switch ON. Since thetransistor Q2 is connected between the positive battery voltage and thegate of the power MOSFET, the gate will be in a high state when thetransistor Q2 is ON, conducting current from the positive batteryvoltage. Since the power MOSFET Q1 is off when the gate is in a highstate, the power MOSFET Q1 will revert to the OFF or nonconducting statewhen the transistor Q2 is ON, and one event that will turn thetransistor Q2 On is the disconnection of the short pin or shunt inconnector 2. The switching time of this solid state relay 1 issufficiently fast that a short pin that is approximately 0.200 inchesshorter than a load pin will provide adequate time for the relay 1 toswitch from an ON to and OFF state before arcing will occur as thelonger load pin is disconnected in the connector 2.

For applications in which a single power MOSFET Q1 is inadequate tocarry sufficient current, plural power MOSFET's can be paralleled. FIG.2 shows a solid state relay 1 in which two power MOSFET's Q1 and Q3 areconnected in parallel between the positive battery voltage and the load.Resistor R6, having the same value as Resistor R5, is connected to thegate of power MOSFET Q3 in the same manner as Resistor R5 is connectedto the gate of power MOSFET Q1. As can be seen from FIG. 2, the sameinput will be applied to the two gates of the two power MOSFET's Q1 andQ3. Additional power MOSFET devices can be paralleled in the samemanner.

Suitable values for the various components shown in FIGS. 1 and 2 arelisted below.

Q1 IRF4905/TO-220 Q2 2N1132 Q3 IRF4905/TO-220 C1 l μF/50 V ceramic Z1 30V or 68 V Zener Diode depending upon B+ Z2 15 volt Zener Diode D1 1N4007D2 BAS70/SOT D3 19TQ15 R1  3 K Ohms R2  10 K Ohms R3  10 K Ohms R4  10 KOhms R5 100 Ohms R6 100 Ohms R7  10 K Ohms R8  1 K Ohms (1%) R9  1 KOhms (1%)

Relevant details of one version the connector 2 are shown in FIGS. 3-6.This connector 2 has two mating connector halves 10, 20, the first ofwhich includes at least male terminal or load pin 12. This male terminalor load pin 12 carries the current between the drain of the power MOSFETQ1 and the load 4, and is terminated to the drain, or to a wire orconductor leading to the drain. A mating female terminal 22 in a secondconnector half 20 is connected to the load 4, either directly orindirectly by means of an electrical conductor. The first connector half10 includes a shunt 16, that protrudes beyond the male terminal 12 andthe housing 14 of the first connector half 10. The shunt 16 must beconnected to relay 1, terminal 87 a and will be connected to the base ofthe PNP transistor Q2, through diode D2 and resistor R9, when this firstconnector half 10 is fully mated with the second connector half 20.

The shunt 16 comprises a deflectable cantilever spring that will alsoengage the female terminal 22 in the second connector half at a contactpoint 18 when the two connector halves 10, 20 are fully mated as shownin FIG. 3. When the shunt 16 is connected to the female terminal 22, theshunt 16 will also be connected to the male terminal 12, andsubsequentially to the drain of power MOSFET Q1. The housing 24 of thesecond connector half includes a protruding section 26 with a rampingsurface 28 that is opposed to the cantilever beam section of the shunt16. As the second connector half 20 is unmated from the first connectorhalf 10, a portion of the shunt 16 adjacent the contact point 18 willengage this ramping surface 28. Continued movement of the two connectorhalves 10, 20, as shown in FIGS. 4 and 5, will disconnect the shunt 16from the female terminal 22. Since the shunt 16 is not otherwiseconnected to the male terminal 12, the shunt 16 will be disconnectedfrom the power MOSFET drain when the shunt 16 is deflected outwardly.The shunt 16 will be disengaged from mating engagement with the femaleterminal 22 before the male terminal 12 is disengaged from the femaleterminal 22. More importantly, the shunt 16 will be disengaged from thefemale terminal 22 before the male and female terminals have reached aposition, as shown in FIG. 6, in which arcing could occur if the currentwere still flowing through the power MOSFET Q1. However, when the shunt16 is first disconnected from the female terminal 22, as shown in FIG.4, the connection between the drain of the power MOSFET Q1 and the baseof the PNP transistor Q2 is disrupted. The transistor Q2 will then turnON turning the power MOSFET Q1 off. The combined switching times of thetransistor Q2 and the power MOSFET Q1 is less than the time intervalbetween disengagement of the shunt from the female terminal and relativemovement of the male terminal to a point, shown in FIG. 5, at whicharcing can first occur as the two connector halves are unmated. FIG. 6shows that when the two connector halves are fully unmated, the shunt 16will not be connected to the load pin 12. When the two connectors aremated, a current path between the shunt 16 and the load pin 12 will onlybe established when the connector halves have reached the position shownin FIG. 3, at which point the power MOSFET can be switched to the ONstate and current will flow through load pin 12. The MOSFET (relay 1)cannot be switched to the ON state unless shunt 16 is engaged at contactpoint 18 as shown in FIG. 3. Current will flow only after the male andfemale terminals have passed the arc susceptible region shown in FIG. 5.

An alternative connector configuration that can be used with this solidstate relay 1 is illustrated by the mating terminals shown in FIGS. 7Aand 7B. This configuration employs two male terminals which can be inthe form of blades or pins or other conventional configurations. Alonger load pin 32 is connected to relay terminal 87. A shorter pin 34is connected to the relay terminal 87 a. When both the pins 32 and 34are connected to a mating female terminal, the relay terminal 87 a isconnected to the relay terminal 87. The load male terminal or terminals32 is longer than the separate short male terminal 34 that is connectedthrough diode D2 to the base of the monitoring transistor Q2. If thelength of the long or load terminal exceeds the length of the shortterminal by approximately 0.200 inch, there will be a sufficientinterval between disconnection of the short terminal 34 from the femaleor receptacle terminal and disconnection of the longer load terminal 32or terminals to permit the transistor Q2 to switch ON and the powerMOSFET Q1 to switch to the OFF state before the load pin 32 reaches aposition in which arcing might occur. Thus, the longer load pin 32 willnot be carrying current when it reaches an arcing position. Theterminals shown in FIG. 7 are only representative of the configurationsthat could be employed with this arc prevention apparatus.

FIGS. 8 and 9 show a relay apparatus incorporating the elements of thisinvention in a standard relay configuration. Relay 1 includes pins 30,85, 86, 87, and 87 a in a standard pin configuration conforming to ISO7588-2. Other standard pin configurations are also possible. Relay 1also includes a printed circuit board 40 on which the other circuitcomponents can be mounted. This configuration also provides thecapability of biasing the Emitter/Base junction of the PNP transistor Q2between 0.1V and 0.6V. by replacing resistor R9 with a resistor of anappropriate value. As shown in FIG. 9, this relay includes two powerMOSFET's Q1 and Q3, so this embodiment will conform to the circuit shownin FIG. 2. It should be understood that only a single power MOSFET canbe employed in a solid state relay of this type or additional powerMOSFET's can be paralleled. The power MOSFET's Q1 and Q3 are bonded to aconventional heat sink 50, and the assembled package shown in FIG. 8 canbe epoxied or inserted into an outer cover, not shown.

FIGS. 1-7 demonstrate the use of a solid state relay in accordance withthis invention in which the relay is used to prevent arcing when anelectrical connector is mated or unmated. Solid state relays used toprevent arcing can be assembled as shown in FIGS. 8 and 9. Solid staterelays of this type can also be used for other applications. Forexample, the solid state relay, shown in the schematic of FIG. 10, canbe substituted for a conventional electromechanical relay having thesame rating. Pin 30 would be connected to the positive potential, pin 87would be connected to the load and pin 86 would be connected to adisconnectable ground, such as an external switch. A standardelectromechanical relay used in the same circuit would have the sameelectrical connections. The battery overvoltage protection provided byZener Diode Z1 and the protection of the relay from transients providedby free wheeling diode D1 could be provided elsewhere in the circuit inwhich the relay is used. FIG. 11 shows a version of a solid state relayin which the Zener Diode Z1 and the freewheeling diode D1 areincorporated in the relay in much the same manner as the schematics ofFIGS. 1 and 2. The embodiment of FIG. 11 would, however, require aground connection 85 that would not normally be available for standardelectromechanical relays of this type.

The standard pin configuration for an ISO 7588-2 mini relay is shown inFIG. 12. Primary dimensions are shown in inches with equivalentdimensions in mm. also shown in the view. Each of the versions of thesolid state relay 1 depicted herein can be implemented with the pinconfiguration shown in FIG. 12. These versions of the solid state relay1 can also be implemented in the pin configuration for a standard miniSAE J1744 relay as shown in FIG. 13. These are the two standard relayconfigurations used for automotive electrical systems, and the smartsolid state relay 1 of this invention is basically compatible with eachstandard relay.

FIG. 14 is a simple schematic for a standard Form A relay. A Form Arelay has one switchable output, and the smart solid state relay 1 ofthis invention has the functionality of a standard Form A relay. FIG. 15is a simple schematic for a relay having a switched output pin 87 and asense pin 87 a. It is this pin configuration with which the smart solidstate relay 1 can be used to prevent arching as shown in FIGS. 1-7. Therelays of FIGS. 1 and 2 will provide the means to shut off current tothe load pins that remain connected to relay output 87 after the shortpin or shunt connected to relay sense pin 87 a has been disconnected.

An important advantage of this invention is that it can be essentiallyadapted to standard relay configurations. It should be understood,however, that representative embodiments of this invention, whichincluded this advantage, are not the only versions of this inventionthat would be apparent to one of ordinary skill in the art. Therefore,this invention is defined by the following claims and is not limited tothe representative embodiments depicted herein.

1. A solid state relay comprising: a power MOSFET for switching currentto a load, the MOSFET including a source, a gate and a drain; the gatecomprising an active low gate for controlling source to drain currentthrough the power MOSFET, the power MOSFET being isolated from groundpotential except through the gate when the solid state relay isconnected between a positive battery voltage and a load; and a circuitfor applying a gate input to shut off source to drain current when avoltage drop between the MOSFET source and drain exceeds a referencevoltage; wherein the circuit and the active low gate are configured tofloat in the absence of an active low input to the gate, and are nottied to ground, to prevent leakage when the power MOSFET is commanded toa nonconducting state OFF state by removal of an low gate input.
 2. Thesolid state relay of claim 1 wherein the circuit for applying a gateinput to shut off source to drain current comprises a bipolar transistorthat turns on to shut off source to drain current when the voltage dropbetween the MOSFET source and drain exceeds a reference voltage.
 3. Thesolid state relay of claim 2 wherein the bipolar transistor includes anemitter base junction that monitors the voltage drop between the MOSFETsource and drain.
 4. The solid state relay of claim 3 wherein theemitter base junction is biased by a diode and resistance between theMOSFET drain and the bipolar transistor base, by a resistance betweenthe MOSFET source and the bipolar transistor base and by a resistancebetween the diode and the MOSFET gate.
 5. The solid state relay of claim4 including a disconnectable contact between the MOSFET drain and thebipolar transistor base, disconnection of the disconnectable contactfrom a load turning the bipolar transistor ON and the power MOSFET off.6. A solid state Form A relay including a first relay terminal (30)connectable to a high battery voltage potential, a second relay terminal(87) connectable to the high side of a load, and a third relay terminal(86) comprising a signal input terminal, the solid state Form A relayincluding circuitry comprising: a power MOSFET having a source connectedto the first relay terminal (30) and a drain connected to the secondrelay terminal (87); the power MOSFET including a gate connected to thethird relay terminal (86), the power MOSFET being turned to an On stateby an active low input applied to the third relay terminal (86); a pullup resistor connected between the first and third relay terminal (30 and86); a voltage sensing circuit comprising means for sensing source-drainvoltage drop when the power MOSFET is in an ON state, the voltagesensing circuit being connected to the gate so that when thesource-drain voltage drop exceeds a reference level, the power MOSFET isturned to an OFF state; and the solid state Form A relay circuitryfloating relative to ground potential when the active low input isremoved from the third terminal (86) so that leakage current between ahigh battery voltage potential and a ground voltage potential iseliminated when the power MOSFET and the solid state form A relay arecommanded to an OFF state by the absence of an active low input at thethird relay terminal (86).
 7. The solid state Form A relay of claim 6wherein the first, second, and third relay terminals are configured in astandard electromechanical relay pin configuration so that the solidstate Form A relay can be substituted for a standard electromechanicalrelay without introducing a leakage path across the solid state Form Arelay when the solid state Form A relay is commanded to an OFF state. 8.The solid state Form A relay of claim 7 wherein the first, second, andthird relay terminals are configured in a standard ISO 7588-2 mini relaypin configuration.
 9. The solid state Form A relay of claim 7 whereinthe first, second, and third relay terminals are configured in astandard mini SAE J1744 relay pin configuration.
 10. The solid stateForm A relay of claim 6 including an additional relay terminal (85)connectable to a ground voltage potential and a zener diode connectedbetween the first and the additional relay terminals.
 11. The solidstate Form A relay of claim 10 including a free wheeling diode connectedbetween the second and the additional relay terminals.
 12. The solidstate Form A relay of claim 6 including an additional relay terminal,the additional relay terminal being unconnected to any circuitry in thesolid state Form A relay, wherein the first, second, third and theadditional relay terminals are configured in a standardelectromechanical relay pin configuration so that the solid state Form Arelay can be substituted for a standard electromechanical relay withoutintroducing a leakage path across the solid state Form A relay when thesolid state Form A relay is commanded to an OFF state.
 13. A solid staterelay including a first relay terminal (30) connectable to a highbattery voltage potential, a second relay terminal (87) connectable tothe high side of a load, a third relay terminal (86) comprising a signalinput terminal, and a fourth relay terminal (87 a) comprising a voltagesense pin, the solid state relay including circuitry comprising: a powerMOSFET having a source connected to the first relay terminal (30) and adrain connected to the second relay terminal (87) and the fourth relayterminal (87 a); the power MOSFET including a gate connected to thethird relay terminal (86), the power MOSFET being turned to an On stateby an active low input applied to the third relay terminal (86); a pullup resistor connected between the first and third relay terminal (30 and86); a voltage sensing circuit comprising means for sensing source-drainvoltage drop when the power MOSFET is in an ON state, the voltagesensing circuit being connected to the gate so that when thesource-drain voltage drop exceeds a reference level, the power MOSFET isturned to an OFF state; the fourth relay terminal (87 a) being connectedto the voltage sensing circuit so that upon disconnection of the fourthrelay terminal (87 a) from a voltage at the second relay terminal, thepower MOSFET is turned to the OFF state; and the solid state relaycircuitry floating relative to ground potential when the active lowinput is removed from the third terminal (86) so that leakage currentbetween a high battery voltage potential and a ground voltage potentialis eliminated when the power MOSFET and the solid state relay arecommanded to an OFF state by the absence of an active low input at thethird relay terminal (86).
 14. The solid state relay of claim 13 whereinthe first, second, third and fourth relay terminals are configured in astandard electromechanical relay pin configuration so that the solidstate relay can be substituted for a standard electromechanical relaywithout introducing a leakage path across the solid state relay when thesolid state relay is commanded to an OFF state.
 15. The solid staterelay of claim 14 wherein the first, second, third and fourth relayterminals are configured in a standard ISO 7588-2 mini relay pinconfiguration.
 16. The solid state relay of claim 14 wherein the first,second, third, and fourth relay terminals are configured in a standardmini SAE J1744 relay pin configuration.